Currently, there is a high demand for transgenic plants that express biotechnologically important protein products at a high or inducible level. Cis-acting sequences from viruses such as CMV have been used successfully in plants to drive expression of various genes. However, these sequences are often prone to rearrangement within the plant cell, and thus exhibit genetic instability. Furthermore, plant mechanisms of gene silencing often reduce, eliminate, or otherwise alter the expression of genes regulated by viral cis-acting elements, reducing the usefulness of such elements.
Ubiquitin is one of the most highly conserved proteins in eukaryotes and can be found throughout the plant body. Many polyubiquitin genes are expressed constitutively (Kawalleck et al. (1993) Plant Mol. Biol. 21:673-684), whereas others are expressed in a tissue-preferred manner (Callis and Bedinger (1994) Proc. Natl. Acad. Sci. U.S.A. 91:6074-6077; Plesse et al. (1997) Mol. Gen. Genet. 254:258-266), or are induced in response to environmental signals (Christensen and Quail (1989) Plant Mol. Biol. 12:619-632; Liu et al. (1995) Biochem. Cell Biol. 73:19-30).
Promoters from ubiquitin have been shown to drive reporter gene expression in transformed cells and plants. These promoters have been isolated from Arabidopsis thaliana (Callis et al. (1990) J. Biol. Chem. 265:12486-12493), sunflower (Binet et al. (1991) Plant Sci. 79:87-94), tobacco (Genschick et al. (1994) Gene 148:195-202; Plesse et al. (1997) Mol. Gen. Genet. 254:258-266), and maize (Christensen et al. (1992) Plant Mol. Biol. 18:675-689).
Maize ubiquitin promoter-based vectors have been developed which result in high-level expression of foreign genes in a number of monocots, including rice, wheat, sugarcane, maize, barley, Pennisetum, Panicum, and Lemna (Christensen and Quail (1996) Transgenic Research 5:213-218; U.S. Pat. Nos. 5,510,474; 5,614,399; 6,020,190; and 6,054,574), including expression of an herbicide resistance gene in rice (Toki et al. (1992) Plant Physiol. 100:1503-1507). A polyubiquitin promoter from potato (Solanum tuberosum) has been isolated and fusion transgenes with this promoter were introduced back into potato, resulting in constitutive production of the transgene in tuber peel, and inducible expression in tuber tissue and leaves (Garbarino et al. (1995) Plant Physiol. 109:1371-1378). In addition, rice polyubiquitin promoters have been shown to drive strong constitutive expression in transformed rice plants (U.S. Pat. No. 6,528,701; Wang et al. (2000) Plant Sci. 156:201-211).
The identification and isolation of regulatory elements useful for strong or inducible expression of genes in microorganisms and plants would be beneficial in the development of commercial varieties of transgenic plants.